1. Field of the Invention
The present invention is broadly concerned with improved jib units of the type which may be mounted on a primary boom to allow a user to lift loads and do other work from a personnel bucket secured to the boom. More particularly, the invention is concerned with such detachable, boom-mounted jib units having a significantly increased range of motion and useful working envelope by provision of a doubly articulating assembly for selective movement of the jib unit when positioned on a boom. Double articulation is provided by means of first and second piston and cylinder assemblies pivotally coupled between the base of the jib unit and the moveable jib arm.
2. Description of the Prior Art
Arial booms are used in a variety of contexts in order to raise and lower heavy loads and allow workers to operate from boom-mounted personnel buckets and the like. For example, a vehicle-mounted arial device typically includes a boom which can rotate, pivot up and down, and extend. The boom assembly generally has a lower boom mounted to the vehicle and an upper boom which articulates relative to the lower boom. The tip of the upper boom carries a working platform such as a bucket or basket from which workers can perform various jobs. It is common for this type of arial device to be used in situations requiring the raising and lowering of heavy loads. This task is usually carried out by means of a pivotal jib unit mounted on the tip of the upper boom, with the jib unit supporting a winch assembly.
U.S. Pat. No. 6,044,991 illustrates such an arial boom device. The jib unit described in the '991 patent has a single piston and cylinder assembly for articulation of the jib arm. This inherently limits the range of motion and useful work envelope of the jib unit. A problem commonly encountered in such cases is that when lifting heavy loads, the jib unit must be positioned in such a location that the load cannot be elevated to a convenient work height. Thus, the user may be forced to bend over the edge of the bucket in order to manipulate the load. This is not only inconvenient, but effectively lessens the loads which the user may be able to handle.
The prior art describes a vast array of boom designs, both in mobile vehicle-mounted booms and in static units. Some of these designs are illustrated in Patents and Patent Publications Nos. U.S. Pat. Nos. 4,150,754, 3,774,389, 3,917,088, 4,464,093, 4,660,729, 4,927,315, 6,860,396, 4,027,772, 3,924,776, 4,094,422, 3,991,886, 3,487,964, 4,252,213, 4,222,491, 4,368,824, 4,861,224, 4,456,093, 2004/0262078, 2004/0164042, 3,819,922, 6,843,383, 5,337,854, 4,178,591, 4,582,206, 5,826,859, 4,828,124, 4,053,060, 5,337,854, 4,838,381, 4,063,649, 2,988,040, 4,759,685, GB2148983, GB1400402, DE2807518, DE3633582, DE29801914, DE29619474, DE4203186, DE3112586, SE521093, WO94/27906, EP1512388, EP1000802, and EP0513939.